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For further information, see CMDT Part 6-48: Furunculosis (Boils) & Carbuncles

Key Features

Essentials of Diagnosis

  • Extremely painful inflammatory swelling based on a hair follicle that forms an abscess

  • Coagulase-positive Staphylococcus aureus is the causative organism

  • Predisposing condition (diabetes mellitus, HIV disease, injection drug use) sometimes present

General Considerations

  • A furuncle (boil) is a deep-seated infection (abscess) involving the entire hair follicle and adjacent subcutaneous tissue

  • The most common sites of occurrence are the hairy parts exposed to irritation and friction, pressure, or moisture

  • Because the lesions are autoinoculable, they are often multiple

  • A carbuncle consists of several furuncles developing in adjoining hair follicles and coalescing to form a conglomerate, deeply situated mass with multiple drainage points


  • Predisposing cause usually not found

  • However, diabetes mellitus (especially diabetics using insulin injections), injection drug use, allergy injections, and HIV disease all increase the risk of staphylococcal infections by increasing the rate of carriage

Clinical Findings

Symptoms and Signs


  • Rounded or conical abscesses on the hairy parts exposed to irritation and friction, pressure, or moisture

  • Lesions are often multiple and pain and tenderness may be prominent

  • Lesions gradually enlarge, become fluctuant, and then soften and open spontaneously after a few days to 1–2 weeks to discharge a core of necrotic tissue and pus


  • Consists of several furuncles in adjoining hair follicles and coalescing to form a deeply situated mass with multiple drainage points

Differential Diagnosis

  • Inflamed sebaceous (epidermal inclusion) cyst

    • Suddenly becomes red, tender, and expands greatly in size over 1 to a few days

    • History of prior cyst in the same location, presence of a clearly visible cyst orifice, and extrusion of malodorous cheesy material (rather than purulent material) helps in the diagnosis

  • Acne vulgaris

  • Tinea profunda (deep tinea of hair follicle)

  • Sporotrichosis

  • Blastomycosis

  • Hidradenitis suppurativa (acne inversa)

    • Recurrent tender sterile abscesses in the axillae, groin, on the buttocks, or below the breasts

    • Presence of old scars or sinus tracts plus negative cultures suggests this diagnosis

  • Anthrax

  • Tularemia


Laboratory Tests

  • Leukocytosis may occur

  • Although S aureus is almost always the cause, pus can be cultured, especially in immunocompromised patients, to rule out methicillin-resistant S aureus (MRSA) or other bacteria



  • Systemic antibiotics

    • Trimethoprim-sulfamethoxazole, 160/800 or 320/1600 mg orally twice a day for 10 days or 7 days, respectively, or clindamycin, 300 mg orally three times daily for 10 days, at the time of drainage

      • Have higher cure rates

      • Lower new infection rates

    • Other antibiotic options ...

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